Thalidomide (N-α-phthalimidoglutarimide) is a glutamic acid derivative that was introduced onto the market as a sedative hypnotic in 1956, but was withdrawn in 1961 due to the development of severe congenital abnormalities in babies born to mothers using it for morning sickness. Interest in the agent was reawakened after thalidomide was found clinically effective in the treatment of erythema nodosum leprosum (ENL) and in the treatment of HIV wasting syndrome and various cancers. Mechanistic studies of its ENL activity demonstrated an anti-tumor necrosis factor alpha (anti-TNF-α) action. Specifically, thalidomide enhances the degradation of TNF-α RNA, and thereby lowers its synthesis and secretion. Further studies have defined it to be a co-stimulator of both CD8+ and CD4+ T cells, an inhibitor of angiogenesis via its inhibitory actions on basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF), and an inhibitor of the transcription factor, NFκB.
TNF-α and family members play pivotal roles in a variety of physiological and pathological processes, which include cell proliferation and differentiation, apoptosis, the modulation of immune responses and induction of inflammation. TNF-α acts via two receptors, TNFR1 and 2. The former is expressed in all tissues and is the predominant signaling receptor for TNF-α. The latter is primarily expressed on immune cells and mediates more limited biological responses. The exposure of cells to TNF-α can result in activation of a caspase cascade leading to cell death via apoptosis. Indeed, major cell surface molecules capable of initiating apoptosis are members of the TNF family of ligands and receptors. For example, death-inducing members of the TNF receptor family each contain a cytoplasmic ‘death domain’ (DD), which is a protein-protein interaction motif critical for engaging downstream components of the signal transduction machinery.
Recently, TRAIL, the tumor necrosis factor-related apoptosis-inducing ligand, has been shown to selectively induce apoptosis of tumor cells, but not most normal cells. It is indicated that TRAIL mediates thymocyte apoptosis and is important in the induction of autoimmune diseases. More often, however, TNF-α receptor binding induces the activation of transcription factors, AP-1 and NFκB, that thereafter induce genes involved in acute and chronic inflammatory responses. Overproduction of TNF-α has thus been implicated in many inflammatory diseases, such as rheumatoid arthritis, graft-versus-host disease and Crohn's disease, and it additionally exacerbates ENL, septic shock, AIDS and dementia associated with Alzheimer's disease (AD).
A number of thalidomide analogs optimized to reduce TNF-α synthesis have been designed and synthesized. Primarily, these analogs include structural modifications of the phthaloyl ring or glutarimide ring of thalidomide. In addition, following the demonstration that the anti-angiogenic property of thalidomide is associated with its hydroxylated, open-ring metabolites, syntheses of the hydroxylated and hydrolysis metabolites as inhibitors of angiogenesis or tumor metastasis have been reported. Although extensive studies exist regarding the structure-activity relationships between thalidomide and TNF-α, very little is known about the contribution of the four amide carbonyl groups of thalidomide to its biological activity.